The invention relates to a spreader device for knee joints comprising two parallel support plates which can be inserted between the femur condyles and the tibia and which can be moved apart by means of an adjustment mechanism which includes a housing.
Spreader apparatuses at joints serve for the spreading of an CD exposed joint in order to carry out certain surgical operations on the joint. For knee joints, such apparatuses have two parallel plates which are inserted between the tibia condyles and femur condyles and then moved apart with the aid of an adjustment mechanism.
A spreader apparatus of this kind is manufactured by Sulzer Orthopedics, Inc., 9900 Spectrum Drive, Austin Tex. 7871. The support plates are moved apart by means of a rack in whose teeth a spring pawl latches to secure the achieved spreading height. The vertical adjustment of the rack takes place via a gear which engages into the rack and can be driven via a crank. A disadvantage of this device lies in the fact that the operator does not have any information on the condition of the ligaments.
It is an object of the invention to provide remedial help in this direction with an improved spreader apparatus. This object is achieved with an adjustment mechanism that includes an elastically resilient transmission member to one of the support plates, in order to simultaneously make the effective spreading travel readable on a scale at the housing and the amount of the spreading force readable on a scale at the housing.
The advantage of the invention lies in the fact that the reaction forces of the ligaments can be read directly during spreading and can be compared to a likewise readable spreading height.
The support plates have a rear recess to protect cruciate ligaments which are still present. Since a further recess is provided at the front side of the support plates and since the connection to the adjustment mechanism takes place via a laterally receding web, an insertion of the support plates from the side is possible from the side without a web, for example from lateral, without the patella and the patella ligaments having to be removed from their natural position. The web is guided in an arc to the adjustment mechanism such that its feed axis lies in the sagittal central plane of the support plates. Since the support of a support plate, for example the upper support plate, takes place in this central plane in a hinge joint with a horizontal axis, the upper support plate can tilt away medially or laterally and display the current varus/valgus position between the femur and tibia to the operator. The operator thus has the possibility to examine the working of the ligaments under natural conditions. The support plates are connected to the adjustment mechanism via the webs by releasable plug connections. These plug connections are present a second time in mirror position at the central sagittal plane and are symmetrically designed so that the same support plates can be used for lateral or medial insertion. The releasable plug connection also allows support plates of different size to be used in pairs. Since the outer spacing of the support plates for the insertion of the support plates is limited and since bending deformations of the support plates of too large a kind are unwanted, it can be advantageous if the support plates have ribs and valleys in certain regions which mutually penetrate one another and give the individual support plate greater section modulus against bending.
The actual vertical adjustment of the spreading height takes place using a threaded screw, which is supported at the housing via an elastic spring, and moves a tappet which bears the upper support plate upwardly or downwardly. The thread has the advantage that a fine adjustment of the spreading height and a self-locking of the adjustment mechanism are given.
To take away as little room as possible in the operation area, the adjustment mechanism is designed such that it only projects a little beyond the upper support plate and is provided with a narrow tubular housing whose diameter has room twice in the dimensions of the support plate from lateral to medial. The tubular housing is extended by a rotary sleeve which includes an extension of the threaded screw with respect to rotation in form-locked manner. Contact surfaces are applied to the rotary sleeve so that additional space is only taken up by an applicable tool, for example a fork wrench, during the adjustment. The extension which projects out of the housing in dependence on the stressing of the spring between the threaded screw and the housing allows the spring force to be directly read off a scale of the rotary sleeve.
A further advantage of the design with exchangeable support plates lies in the fact that mono-condylar support plates can be used for only one joint half. However, in such a case, the hinge joint must be blocked or the connection webs must be guided to the adjustment mechanism such that the hinge axis comes to rest in the central sagittal plane of the mono-condylar support plates.
A further application provides for the fastening of cutting blocks to the support plates at predetermined positions in order to carry out resection incisions at bone parts in a position found to be optimum.
The invention is described below with reference to embodiments.